Filter element and hydraulic circuit with such a filter element

ABSTRACT

A filter element has a preferably multilayer structure of a pleated filter medium (3) with a plurality of individual filter pleats (5, 7). Due to an alternating sequence of filter pleats (5) with a first pleat height (h1) and filter pleats (7) with a second pleat height (h2), more effective filter surfaces are available than with filter pleats with a uniform pleat height. Upon throughflow by a fluid to be filtered, a lower surface load for the filter medium (30 and lower passage speed of the fluid are present during filtration. A static loading of the filter medium (3) during operation of the filter is then reduced. A hydraulic circuit has such a filter element (1).

FIELD OF THE INVENTION

The invention relates to a filter element with a pleated filter mediumhaving a preferably multilayer structure, including a plurality ofindividual filter pleats. In addition, the invention relates to ahydraulic circuit of such a filter element.

BACKGROUND OF THE INVENTION

Filter elements of this kind are available commercially. Such filterelements are widely used in conjunction with a variety of fluid systemsfor filtering process fluids, pressure fluids such as hydraulic oil, aswell as liquid fuels and lubricants for preparing fluid media and thelike.

In many cases, only a limited amount of usable space is available influid systems, in which the filter elements are used, for installing orremoving system parts that contain the relevant filter cartridge-typefilters. On the other hand, a filtering surface of sufficient sizeprovided by the filter element is required to filter correspondinglylarge fluid flows.

To provide a sufficiently large filtering surface, the known filterelements available on the market have a typically zig-zag-shaped foldedor pleated filter medium composed of multiple layers of various filtermaterials. During manufacture, the filter medium is fed through acutting device, in which the edge of the filter medium is cut to sizebefore it is conveyed further to a folding machine, in which the zig-zagshape or the pleating, having a plurality of individual filter folds, isformed. During the further course of manufacture, the customized filtermedium is separated into sections, which are shaped to form a tubularbody, thereby forming the filter element.

In the standard filter element solution described above, all the filterfolds routinely have the same insertion height. Depending on theflow-through conditions, the multiple filter folds disposed adjacent toone another may be displaced toward one another due to their flexibilityor resilience, and thus come into direct contact with one another alongtheir effective filtering surface. This displacement results in a typeof “blocking” of the element material in this contiguous region, sincethe medium to be filtered is then no longer able to reachuninterruptedly all the filter folds of the element structure. Theresult is that the remaining filter folds spread apart from one another,are not blocked, and are increasingly perfused by the fluid to becleaned of the particle contaminants. As a consequence, the flowvelocity rises, and the surface load on these folds of the filter mediumis increased. Since the multilayer filter medium is routinely formedfrom individual nonwoven filter medium of individual fibers, this loadincrease results in an increased discharge of fiber material from thefleece-composite material (migration), which, in turn, reduces theservice life of the filter element.

One great challenge the filtration technology must now face is the factthat, due to environmental regulations, the hydraulic fluids to becleaned using primarily such filter elements may no longer include anymetal additives, in particular, any environmentally harmful zincadditives. As a result, the electrical conductivity of the hydraulicfluid is reduced. Due to this reduced conductivity, electrostaticcharges, as they routinely occur during flow-through of the medium, canno longer be effectively dissipated via the hydraulic fluid, as in thepast. As a result of this inability, discharge processes may occur inthe filter element and may occur in the form of discharge flashes. Thoseflashes routinely destroy the sensitive filter medium in the same waythat they promote the oil aging of the hydraulic fluid(s). Particularlyin the case of in-tank applications of the filter element, in which thefilter element is used in closed tank units, such discharge flashes mayincrease the danger of explosion.

To counter these effects, the prior art (DE 10 2004 005 202 A1) hasproposed providing, in conjunction with a filter element having a filtermedium, which extends between two end caps. Each end cap is connected toan assignable end region of the filter medium and is supported on atleast one side by a support tube. At least one of the end caps and/or atleast one end region of the filter medium includes a contacting deviceand/or each end cap itself or parts thereof are designed to beconductive, to thereby ensure a dissipation of the electrostatic chargesroutinely occurring during operation of the filter element. The chargegenerated on the filter medium as a result of the above describedtribological effects may thus drain off at a ground point or ground sitevia the contact device and/or via each end cap. The “controlleddissipation” in this regard has proven to be very effective. However,maintaining the upstream contact device requires increased material andmanufacturing expenditure, which, accordingly, reflects negatively interms of the manufacturing costs of the element.

An alternative approach has been described in WO 2009/089891 A1, whichshows a filter element solution without the use of a contact device. Inthis known solution, a manufacturing material is selected for the filtermedium, the potential difference of which to the fluid to be cleaned isminimal, depending on the respective selected cleaning task. Inparticular, filter solutions in this case are addressed, in which partsof the filter medium exhibit such different potentials relative to oneanother and/or to the fluid to be cleaned (hydraulic fluid), that, atleast in part, they cancel each other out during the filtrationoperation, or in which a targeted discharge back into the hydraulicfluid is sought, or in which a return of electrical charge to the entirefilter medium is provided using a charge equalization layer in thefilter media system. Thus, with this known solution, estimating thepotential produced between two interacting components, i.e., between oiland filter medium, in a normal application, is possible in accordancewith a known electrical voltage series for various filter materialsprovided for the filter medium (filter fleece). In this known, veryadvantageous solution, very little charge is generated, in principle, sothat the problem of having to discharge the former at a ground point asshown in the above description, via a discrete contact device, does noteven arise.

Nevertheless, here too, when selecting the filter insert materials inquestion, based on the aforementioned tribological voltage series, acorrespondingly high expense must be incurred, both from the standpointof pre-development as well as from the standpoint of material supply, toobtain positive results, which again increases the manufacturing costsof the filter element.

In addition, all of the known solutions described above have in commonthe fact that the disadvantageous “blocking” effect may occur as aresult of displacement of free filter fold ends due to the flow-throughof the fluid.

SUMMARY OF THE INVENTION

Against this background, an object of the invention is to provide animproved filter element and a hydraulic circuit having such a filterelement, in which, given a cost-effective and functionally reliabledesign, the danger of undesired electrostatic discharge is minimized.

This object is basically achieved by a filter element and a hydrauliccircuit having such filter element where the filter element has agreater effective filtering surface available due to an alternatingsequence of filter pleats having a first pleat height and of filterpleats having a second pleat height, than is the case with knownsolutions with filter pleats having a uniform pleat height. To a personof average skill in the field of filtration technology, surprisinglysuch configuration of filter pleats and a corresponding flow-through ofa fluid to be filtered results in an overall lower surface load to thefilter medium, and in a lower velocity at which the fluid passes duringfiltration, which, in turn, assists in significantly reducing the staticcharge of the filter medium during operation of the filter.

A stabilization takes place as a result of the aforementionedconfiguration of filter pleats, wherein the filter pleats of the secondpleat height reinforce the filter pleats of the first pleat height.Thus, a pressure-resistant and functionally reliable structure for thefilter medium is achieved via the filter pleats of the differing heightcharacteristic. In addition, the filter pleats of the first pleat heightare maintained in their original configuration during operation of thefilter element and, in this way, have a particularly large effectivefiltering surface facing the dirt side or the clean side. The opensurface facing the dirty side or the clean side is therefore larger thanin the case of conventional filter elements having a uniform pleatheight over the entire circumference, which are routinely prone to theaforementioned “blocking”. Hence, this structure allows the fluid in thesolution according to the invention to more easily penetrate the filtermedium, which results in a filter element having improved filterefficiency over the life of the filter, and having a longer servicelife.

Efforts are frequently made in filtration technology to optimize thefilter element with its filter medium such that a high flow-throughefficiency, in addition to a high cleaning efficiency, is achieved byusing as many individual filter pleats of the same insertion height aspossible in the element. Here, a different approach is taken by simplydispensing with a plurality of individual pleats of the same height and,therefore, with theoretically available filtering surface. By reducingthe height of the filter pleats having the second pleat height, thelateral boundary surfaces of the adjoining filter pleats of the firstfilter height are kept free for a flow-through. In this way, thosesurfaces increase the effective filtering surface and, therefore, theflow-through efficiency and the separating efficiency of the filtermedium. A lower flow-through velocity for the fluid has been shown to beachieved as a result of the associated, on the whole lower, surface loadto the filter medium with a comparable flow-through efficiency, ascompared to known solutions. The result is that electrostatic charges donot even occur or occur at least only to a reduced extent, so that thecharges need not be overcome via additional structural measures, such asa contact device or a selective choice of filter materials havingdifferent potentials.

In spite of these advantageous properties, the solution according to theinvention can be appropriated combined with an additional conductivecontact device and/or with the antistatic design of the filter mediumdescribed above, which will be described in greater detail below.

Preferably, the filter pleats having the second pleat height are atleast half as high as the filter pleats having the first pleat height.In this context, tests were able to show that an optimum exists if thesecond pleat height is 85% to 95%, preferably 90%, of the first pleatheight. For this range of the pleat height, an optimal density of thefilter medium in conjunction with a maximum free surface of the filtermedium is at the lowest electrostatic charge possible.

Alternatively, the filter pleats having the second pleat height may behalf as high as the pleats having the first pleat height.

Particularly preferred, the transitions of the filter pleats have auniform size, preferably having uniform curvature radii. Due to thestandardized curvature radii, the filter material is protected duringthe forming of the pleats. Breaks do not form in the filter material,which would adversely affect the filtration efficiency of the filterelement. Also avoided as a result are sharp-edged transitions and tips,at which electrical voltage spikes could form, with the negative resultthat electrical voltage is released into the fluid, in particular in theform of hydraulic fluid(s).

In addition, straight or planar sections of the filter pleats may eachbe spaced apart from one another. This spacing ensures that the fluidpresent on the dirt side is present over the entire surface of thefilter medium at an approximately uniform pressure. Thus, the entiresurface of the filter medium bears approximately the same surface load.In particular, this structure avoids turbulences during the flow-throughof the filter medium, which otherwise promote the electrostatic charge.

In one particularly advantageous embodiment, the pleated filter mediumhas a tubular design, and the straight or planar sections of the filterpleats each extend in a plane, in which a central longitudinal axis ofthe filter element is also situated. This configuration yields aparticularly high packing density of the filter medium with a pluralityof filter pleats of varying pleat heights.

The filter medium may include multiple layers of filter fleece or otherlayers of filtering media. The layers preferably have different electronrelease properties and/or electron receiving properties. A filter layerpreferably releases electrons to the same extent that they are receivedby the other filter layer. Upon flow-through of the filter medium, theproperties present are then essentially electrically neutral.

As viewed in an axial top view of the filter medium and from the dirtside or the clean side, the filter pleat having the second pleat height,which is delimited in each case by an adjacently disposed filter pleathaving the first pleat height, has a w-type pleat configuration.

Due to the w-type pleat configuration, an open holding space for fluidis formed on the clean side or on the dirt side in the manner of aphantom cylindrical segment between two adjacently disposed filterpleats having the first pleat height. Those two pleats of the firstpleat height delimit a filter pleat having the second pleat height. Theholding space results in an equalization, and preferably a reduction, ofthe flow velocity of the fluid through the filter element duringoperation of the filter element.

Also due to the w-type pleat configuration, which under normalconditions is perfused with a fluid contaminated with particlesresulting in an electrostatic charging of the filter element, thischarging is reduced during operation of the filter element as a resultof the reduction of the fluid flow velocity caused by the respectiveholding space.

The hydraulic circuit according to the invention includes a filterelement of the aforementioned kind and a ground for the circulatingfluid. The filter element as described above is designed such that itgenerates less charge in the fluid than is degradable by grounding thehydraulic circuit with the use of its components.

Other objects, advantages and salient features of the present inventionwill become apparent from the following detailed description, which,taken in conjunction with the annexed drawings, discloses a preferredembodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings that form a part of this disclosure:

FIG. 1 is a front-end top view of the filter element according to anexemplary embodiment of the invention; and

FIG. 2 is an enlarged, partial top view of a lower segment portion ofthe filter element according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of simplicity, the filter element is depicted wholly or inpart in FIGS. 1 and 2, in each case only in a front-end top view. Thefilter element as such routinely has a cylindrical shape, as is shown,for example, in the previously cited prior art documents DE 10 2004 005202 A1 and WO 2009/089891 A1. In this respect, in terms of the overallconfiguration of the filter element, reference is made to the relevantdocuments.

FIG. 1 also shows a filter element 1 including a filter medium 3 havinga multilayer structure. The filter medium 3 includes first and secondfilter pleats 5, 7 of varying first and second pleat heights h1, h2,respectively. The filter pleats have a sequence of first filter pleats 5having a first pleat height h1 alternating with second filter pleats 7having a second pleat height h2. In this way, more effective filteringsurface is available than in the case of filter elements, in which thefilter pleats have a uniform pleat height. Upon flow-through of a fluidto be filtered, the result is a lower surface load to the filter medium3 and a lower flow-through velocity of the fluid during filtration.Thus, the static charging of the filter medium 3 and of the filteredfluid is reduced during filter operation.

As can be seen in FIG. 1, the filter pleats 5 having the first pleatheight h1 are approximately twice as high as the filter pleats 7 havingthe second pleat height h2. In addition, the filter pleats 5, 7 areprovided with transitions 9, which have curvature radii KR of a uniformsize. As a consequence of this, the straight or planar sections 11 ofthe filter pleats 5, 7 are each spaced apart from one another. Thepleated filter medium 3 has a tubular design. The straight sections 11of the filter pleats 5, 7 each extend in a plane 13, in which a centrallongitudinal axis MLA of the filter element is also located. In thisway, the planar sections 13 of the filter pleats 5, 7 are spaceduniformly apart from one another. Thus, an adhesion or bonding of thefilter pleats 5, 7 cannot occur, which would adversely affect thefiltering efficiency.

The filter medium 3 has multiple filter layers, made, in particular, offilter fleece. These may differ in terms of their electron releaseproperties or electron receiving properties. In this way, for example,for electrical charges can be accumulated by fluid in one layer of thefilter medium 3 to then be released in another layer.

As viewed in the axial top view of the filter medium 3 and from theclean side R, the filter pleat 7 having the second pleat height h2,which is delimited in each case by an adjacent filter pleat 5 having thefirst pleat height, forms a w-type pleat configuration. The individualfilter pleats 5, 7 of varying pleat heights h1, h2 are spaced apart fromone another to form the w-type pleat configuration. In this way, finefiltration regions are formed on the bottom 15 of the filter medium 3disposed on the clean side.

Due to the w-type pleat configuration, an open holding space 17 forfluid is formed on the clean side R in the manner of a virtualcylindrical segment between two adjacently disposed filter pleats 5having the first pleat height h1, which delimit a filter pleat 7 havingthe second pleat height h2, the holding space resulting, duringoperation of the filter element 1, in an equalization and, preferably ina reduction, of the flow velocity of the fluid through the filterelement 1.

Finally, due to the ω-type pleat configuration, which under normalconditions is perfused with a fluid contaminated with particles from theinner side 19 to the outer side 21, resulting in an electrostaticcharging of the filter element 1, this charging is reduced duringoperation of the filter element as a result of the reduction of thefluid flow velocity caused by the respective holding space 17.

In FIG. 1, the filter element 1 is delimited toward the inner side 19 aswell as to the outer side 21 by phantom circles 23, 25. However, theinner contour 23 and/or the outer contour 25 of the filter element 1 mayhave shapes other than circular, in particular, triangular, rectangular,polygonal or ellipsoidal cross-sections. In addition, equally thick orReuleaux triangles (not depicted) are possible as inner and/or outercross-sectional shapes. Since the filter element 1, as viewed in thedirection in FIG. 1, is perfused from the inside to the outside, acorresponding support tube known in the prior art, and therefore notfurther depicted, having a perforation as a fluid passage, may bepresent on the outside 25 of the filter element.

FIG. 2 shows an enlarged image detail of a lower cylindrical segmentaccording to FIG. 1 with a single ω-pleat depicted in a darkened color.As illustrated in FIG. 2, the ω-pleat in this case is delimited from theadjacent adjoining ω-pleats by two virtual parting lines T1 and T2. Theoverall structure of the ω-pleat is uniform. The inner side walls 28 ofthe filter pleats 5 facing one another and the outer side walls 30 ofthe middle filter pleat 7, which adjoin in pairs the respectivelyassociated side walls 28, define, relative to one another, a filterpleat spacing that corresponds essentially to the pleat spacing which isbounded by the inner side walls 32 of the filter pleat 7 in the centerof the ω-pleat configuration. The pleat spacing between the inner sidewalls 34 of each filter pleat 5 is similarly bounded. Thus, theaforementioned pleat spacings in alternating sequence between the filterpleats 5 and the center filter pleats 7 of each and every ω-pleatstructure are identical, and all side walls 28, 30, 32 extend virtuallyparallel to one another.

In terms of charging technology, the adjacent side walls 36 and 38 ofthe filter pleats 5, 7 facing one another form a type of platecapacitor, which are accordingly able to absorb the charges in the freefluid medium flow. Due to the regular configuration of the individual,ω-shapes adjacent one another in the pleat structure, an equalization ofthe electrical potential occurring over the entire surface of the filtermaterial is achieved, so that charge jumps are avoided. Thus, even inthe case of electrostatic charging, dangerous discharge flashes cannotoccur.

To equalize the charge transport within the filter medium, the filterpleat transitions 9 are also equalized. In particular, they formcontinuous return bends. In contrast to the acute-angled pleattransition as shown in the prior art, the transitions of this inventionform no points having voltage spikes. The charge could be abruptlypassed to the hydraulic fluid via the sharp-edged pleating.

The uniformly molded arc contours in the filter material implement atype of spring characteristic, which creates a restoring force, inparticular in the case of pressure spikes in the fluid to be filtered,thus preventing the pressure spikes from exerting a harmful effect onthe medium.

As shown in the representation according to FIG. 2, a fine filtrationspace is also created by the pleat spacings in the lower region of theω-pleat. Together with the center pleat 7, the fine filtration spacereinforces the respective ω-element in the pleat structure upon receiptof the particle contaminants.

In this exemplary embodiment, the clean side R is provided on the innerside 19, and the dirty side S is provided on the outer side 21 of thefilter medium 3. The clean side R could also be provided on the outerside 21, and the dirt side S could be provided on the inner side 19 ofthe filter medium 3.

Electrostatic charges in a hydraulic fluid, when flowing through thefilter element 1, are largely avoided by the filter element 1 accordingto the invention. The danger of undesired electrostatic discharges isthen averted. Thus, the filter element 1 may also be used in conjunctionwith hydraulic fluids having low electrical conductivity.

While one embodiment has been chosen to illustrate the invention, itwill be understood by those skilled in the art that various changes andmodifications can be made therein without departing from the scope ofthe invention as defined in the claims.

The invention claimed is:
 1. A filter element, comprising: a tubular andpleated filter medium having a central longitudinal axis; first filterpleats having a first pleat height; second filter pleats having a secondpleat height 85 to 95 percent of said first pleat height, said first andsecond pleats being arranged in an alternating sequence providing agreater effective filtering surface than if said pleated filter mediumhad filter pleats having a uniform pleat height of said first pleatheight, providing a corresponding flow-through of a fluid to be filteredresulting in an overall lower surface load to said filter medium andproviding a lower flow-through velocity of fluid during filtration,thereby resulting in a reduced static charge of said filter mediumduring filter element operation; and planar sections of said filterpleats each extending in a plane containing an entire length of saidcentral longitudinal axis.
 2. A filter element according to claim 1wherein said second pleat height is 90 percent of said first pleatheight.
 3. A filter element according to claim 1 wherein said filterpleats have transitions with curvature radii of a uniform size.
 4. Afilter element according to claim 1 wherein said planar sections of eachof said first and second filter pleats are spaced apart from oneanother.
 5. A filter element according to claim 1 wherein said filtermedium comprises multiple layers of filter material having at least oneof varying electron releasing properties or varying electron receivingproperties.
 6. A filter element according to claim 1 wherein each saidsecond pleat is delimited on each side thereof by an adjacent one ofsaid first pleats defining a ω-shaped pleat configuration as viewed froman axial top and one of an inner side or an outer side of said filtermedium.
 7. A filter element according to claim 6 wherein open holdingspaces are formed on at least one of said inner or outer side of saidfilter medium of a virtual cylindrical segment between two adjacent onesof said first filter pleats, said holding spaces equalizing and reducingflow velocity of fluid through said filter medium during filteringoperation.
 8. A filter element according to claim 1 wherein said planarsections are spaced uniformly apart from one another.
 9. A hydrauliccircuit, comprising: a filter element including a tubular and pleatedfilter medium having a central longitudinal axis, first filter pleatshaving a first pleat height, second filter pleats having a second pleatheight less than said first pleat height, said first and second pleatsbeing arranged in an alternating sequence providing a greater effectivefiltering surface than if said pleated filter medium had filter pleatshaving a uniform pleat height of said first pleat height, providing acorresponding flow-through of a fluid to be filtered resulting in anoverall lower surface load to said filter medium and providing a lowerflow-through velocity of fluid during filtration, thereby resulting in areduced static charge of said filter medium during filter elementoperation, and planar sections of said filter pleats each extending in aplane containing an entire length of said central longitudinal axis; anda grounding for circulating fluid in said filter element.
 10. Ahydraulic circuit according to claim 8 wherein said planar sections arespaced uniformly apart from one another.